Modern room-temperature magnetic refrigeration (MR) systems may employ an Active Magnetic Regenerator (AMR) cycle to perform cooling. An early implementation of the AMR cycle can be found in U.S. Pat. No. 4,332,135, the entire disclosure of which is incorporated herein by reference. The AMR cycle has four stages, as shown schematically in FIGS. 1a to 1d. The MR system in FIGS. 1a to 1d includes a porous bed of magnetocaloric material (MCM) 190 and a heat transfer fluid, which exchanges heat with the MCM as it flows through the MCM bed 190. In FIGS. 1a to 1d, the left side of the bed is the cold side, while the hot side is on the right. In alternative embodiments, the hot and cold sides may be reversed. The timing and direction (hot-to-cold or cold-to-hot) of the fluid flow may be coordinated with the application and removal of a magnetic field. The magnet field may be provided by either a permanent magnet, electromagnet, or superconducting magnet.
In an illustrative example of an AMR cycle, FIG. 1a, the first stage of the cycle, “magnetization,” occurs. While the fluid in the MCM bed 190 is stagnant, a magnetic field 192 is applied to the MCM bed 190, causing it to heat. In the magnetization stage shown in FIG. 1a, four valves shown are all closed, preventing fluid flow through the MCM bed 190. The four valves include a cold inlet valve 182, a cold outlet valve 184, a hot outlet valve 186, and a hot inlet valve 188. In FIG. 1b, the second stage of the cycle, “cold-to-hot-flow” occurs. The magnetic field 192 over the MCM bed 190 is maintained, and fluid at a temperature TCi (the cold inlet temperature) is pumped through the MCM bed 190 from the cold side to the hot side. The cold inlet valve 182 and hot outlet valve 186 are open during this stage to facilitate movement of the fluid through the MCM bed 190. The cold outlet valve 184 and the hot inlet valve 188 are closed during this stage. The fluid removes heat from each section of the MCM bed 190, cooling the MCM bed 190 and warming the fluid as it passes to the next section of the MCM bed 190, where the process continues at a higher temperature. The fluid eventually reaches the temperature THo (the hot outlet temperature), where it exits the MCM bed 190 through the hot outlet valve 186. Typically, this fluid is circulated through a hot side heat exchanger (HHEX) 194, where it exhausts its heat to the ambient environment. In FIG. 1c, the third stage, “demagnetization”, occurs. The fluid flow is terminated when the cold inlet valve 182 and the hot outlet valve 186 are closed and the magnetic field 192 is removed. The cold outlet valve 184 and the hot inlet valve 188 are also closed during this stage. This causes the MCM bed 190 to cool further. In FIG. 1d, the final stage of the cycle, “hot-to-cold-flow”, occurs. Here, fluid at a temperature THi (the hot inlet temperature) is pumped through the MCM bed 190 from the hot side to the cold side in the continued absence of the magnetic field 192. In this stage, cold outlet valve 184 and hot inlet valve 188 are open, while cold inlet valve 182 and hot outlet valve 186 are closed. The fluid adds heat to each section of the MCM bed 190, warming the MCM bed 190 and cooling the fluid as it passes to the next section of the MCM bed 190, where the process continues at a lower temperature. The fluid eventually reaches a temperature TCo (the cold outlet temperature) which is the coldest temperature reached by the fluid in the cycle. Typically, this colder fluid is circulated through a cold side heat exchanger (CHEX) 196, where it picks up heat from the refrigerated system, allowing this system to maintain its cold temperature.
A major advantage of the AMR cycle is noted in K. L. Engelbrecht, G. F Nellis, S. A Klein, and C. B. Zimm, Recent Developments in Room Temperature Active Magnetic Regenerative Refrigeration, HVAC&R Research, 13 (2007) pp. 525-542 (hereinafter “Engelbrecht et al.”), the entire disclosure of which is incorporated herein by reference. The advantage is that the span (the temperature at which the heat is exhausted minus the temperature at which heat is absorbed) can be much larger than the absolute value of the temperature change of the magnetocaloric material when the magnetic field is applied (the adiabatic temperature change, Delta-Tad).
The time that it takes to complete execution of the four stages of the AMR cycle is called the cycle time, and its inverse is known as the cycle frequency. The “temperature span” of the MR system is defined as THi−TCi, which is the difference in the inlet fluid temperatures. The AMR cycle is analogous to a vapor compression cycle, where gas compression (which causes the gas to heat) plays the role of magnetization, and where free expansion of the gas (which drops the gas temperature) plays the role of demagnetization. In the vapor compression cycle, the heat transfer fluid changes phase in the CHEX and HHEX to aid in heat transfer. No such phase change need occur in the CHEX and HHEX of the AMR cycle, but a fluid with a high single phase heat transfer coefficient, such as water, may be used. Although FIGS. 1a to 1d illustrate the operation of a single-bed MR system, in alternative embodiments, multiple beds, each undergoing the same AMR cycle, may be combined in a single system to increase the cooling power, reduce the system size, or otherwise improve the implementation of the AMR cycle.